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है”ह”ह

IS 8225 (1987): Measurement of sound absorption in areverberation room [LITD 7: Audio, Video and MultimediaSystems and Equipment]

IS : 8225 - 1987 UDC 534.833.54 : 634.62 ( First Reprint OCTOBER 1997 ) IS0 354 - 1985

Indian Standard

MEASUREMENT OF SOUND ABSORPTION IN

A REVERBERATION ROOM

( First Revision )

( IS0 Title : Acoustics - Measurement of Sound Absorption in a Reverberation Room )

National Foreword

This Indian Standard, which is identical with IS0 354-l 985 ‘Acoustics - Measurement of sound absorption in a reverberation room’, issued by the International Organization for Standardi- ration ( IS0 ), was adopted by the Bureau of Indian Standards~on the recommendation of the 4coustics Sectional Committee and approval of the Electronics and Telecommunication Division Zouncil.

In the adopted standard certain terminology and conventions are not identical with those used n Indian Standards; attention is especially drawn to the following:

Comma ( , ) has been used as a decimal marker, while in Indian Standards the current practice is to use a point ( . ) as the decimal marker.

Cross Reference

In this Indian Standard, the following International Standards are referred to. Please read in their respective place the following Indian Standards:

International Standard Indian Standard

IEC Pub 225 Octave, half-octave and third-octave IS : 6964-l 973 Octave,~half-octave and band fitters intended for-the analysis of sounds third-octave band filters for analysis of and vibrations sound and vibrations

The Technical Committee responsible for the preparation of this standard has reviewed the xovision of the following IS0 standard and has decided that it is acceptable for use in conjunc- :ion with this standard:

IS0 5725-1981 Precision of test methods - Determination of repeatability and reproducibility by inter-laboratory tests.

Adopted 26 October 1987 I

Q May 1988, BIS I

& 6

BUREAU OF INOIAN STANDARDS

MANAK BHAVAN, 9 BAHAOUR SHAH ZAFAR MARG

NEW DEl_Hl 110002

As in the Original Standard, this Page is Intentionally Left Blank

IS : 8225 - 1987 IS0 354 - 1985

0 Introduction

When a sound source operates in an enclosed space, the level

to which reverberant sound builds up, and the subsequent

decay of reverberant sound when the source is stopped, are

governed by the sound-absorbing characteristics of the boun-

dary surfaces and objects within the space. In general, the frac-

tion of the incident sound power absorbed at a surface depends

upon the angle of incidence. In order to relate the reverberation

time of an auditorium, office, workshop, etc. to the noise

reduction that would be effected by an absorbing treatment, a

knowledge of the sound-absorbing characteristics of the sur-

faces, usually in the form of a suitable average over all angles of

incidence, is required. Since the distribution of sound waves in

typical enclosures includes a wide and largely unpredictable

range of angles, it is convenient, for the purposes of stan-

dardization, to take a uniform distribution as the basic con-

dition. If, furthermore, the sound intensity is independent of

location within the room, such a distribution is called a diffuse

sound field, and the sounds reaching a room surface are said to

be at random incidence.

Measurements under reverberant conditions are necessary

because, in this way, the effects of practical mounting con-

ditions can be included. Furthermore, it is the only way to

determine the sound absorption of discrete objects such as

chairs, office landscaping screens, etc.

The purpose of ,this International Standard is to prqmote uni-

formity in the methods and conditions of mea: drement of

sound absorption in reverberation rooms, so that values deter-

mined by different laboratories agree as closely as is possible at

present. In order to improve precision, it may become

necessary to limit further the variability of test conditions. The

sound absorption data determined by the method described

may be used for design calculations. In certain cases, however,

deviations between predicted and measured values of

reverberation time may occur.

It should be emphasized that, in order to attain the above ob-

jectives, a more diffuse sound field than the one which ordinar-

ily exists in most rooms, auditoria, etc. is required, and certain

other constraints, for example on the dimensions of the

reverberation room, are necessary.

1 Scope and field of application

This International Standard specifies a method of measuring

the sound absorption coefficient of acoustical materials used as

wall or ceiling treatments, or the equivalent sound absorption

area of objects, such as furniture, persons or space absorbers,

in a reverberation room. It is not intended for measur-

ing the absorption characteristics of weakly damped

resonators.

The results obtained can be used for comparison purposes and

for design calculation with respect to room acoustics and noise

control.

2 References

IS0 5725, Precision of test methods - Determination of

repeatability and reproducibility by inter-laboratory tests.

tEC Publication 225, Octave, half-octave and third-octave band

filters intended for the analysis of sounds and vibrations.

3 Definitions

For the purpose of this International Standard, the followmg

definitions apply.

3.1 reverberation time: The time that would be required for

the sound pressure level to decrease by @J dl3 after the sound

source has stopped.

The quantity is denoted by jr and is expressed in seconds.

N(?TE - This definition is based on the assumption that, in the ideal

case, there is a linear relationship between the sound pressure level and

time and that the background noise level is sufficiently low.

3.2 equivalent sound absorption area of a room: The

hypothetical area of a totally absorbing surface without diffrac-

tion effects which, if it were the only absorbing element in the

room, would give the same reverberation time as the room

under consideration.

For the empty reverberation room, tnis quantity is denoted by

A, ; for the reverberation room containing a test specimen, it is

denoted by ,-I,. The quantity is expressed in square metres.

3.3 equivalent sound absorption area of a test specimen: The difference between the equivalent sound ab-

3

IS : 8225 - 1987 IS0 354 - 1985

sorption area of the reverberation room with and without the

test specimen.

The quantity is denoted by A and is expressed in square

metres.

3.4 sound absorption coefficient: The change in equi-

valent sound absorption area after placing a test specimen in

the reverberation room, divided by the area of the test

specimen.

It is only defined for a plane test specimen and is denoted

by as.

NOTE - When evaluating the sound absorption coefficient from

measurements in a reverberation room, the results should be denoted

by the subscript “S”. The use of this subscript avoids confusion with

the sound absorption coefficient defined as the ratio of non-reflected-

to-incident sound energy if a plane wave strikes a plane wall at a par-

ticular angle of incidence. This “geometric” sound absorption coef-

ficient is alway: smaller than unity and may therefore be expressed as a

percentage. The sound absorption coefficient evaluated from

reverberation time measurements may have values larger than unity,

for example due to diffraction effects, and as shall not, therefore, be

expressed as a percentage.

3.5 repeatability, r: The value below which the absolute dif-

ference between two single test results obtained using the

same method on identical test material, under the same con-

ditions (same operator, same apparatus, same laboratory and a

short interval of times) may be expected to lie with a specified

probability; in the absence of other indications, the probability

is 95 %

3.6 reproducibility, R: The value belowwhich the absolute

difference between two single test results obtained using the

same method on identical test material, under different con-

ditions (different operators, different apparatus, different

laboratories and different times), may be expected to lie with a

specified probability; in the absence of other indications, the

probability is 95 %.

4 Principle

Measurement of reverberation times in a reverberation room,

with and without the test specimen. From these times, calcu-

lation of the equivalent sound absorption area A of the test

specimen.

In the case of a plane test specimen, the sound absorption

coefficient is obtained by dividing A by its surface area S.

When the test specimen comprises several identical objects,

the equivalent sound absorption area of an individual object is

found by dividing A by the number of objects.

5 Apparatus

The apparatus shall be such that the requirements given in

clause 7 are met.

6 Test arrangement

6.1 Reverberation room and diffusion of sound field

6.1.1 Volume of reverberation room

The volume of the reverberation room shall be at least 150 m3.

For new constructions, the volume shall be approximately

200 m3.

6.1.2 Shape of reverberation room

The shape of the reverberation room should be such that the

following condition is fulfilled:

I max < 1,9 V”3

where

In

I max is the length of the longest straight line which fits within the boundary of the room (for example, in a rec-

tangular room, it is the major diagonal);

V is the volume of the room.

order to achieve a uniform distribution of natural frequen-

cies, especially in the low-frequency bands, no two dimensions

of the room shall be equal or in the ratio of small whole

numbers.

NOTE - In the case of non-rectangular rooms where the test

specimen is placed on the floor, the results will agree more closely with

results from rectangular rooms if the non-vertical walls slant inwards.

6.1.3 Diffusion of sound field

The decaying sound field in the room shall be sufficiently dif-

fuse. In order to achieve satisfactory diffusion, whatever the

shape of the room, the use of stationary, suspended diffusers

or of rotating vanes is, in general, required (see annex A).

6.1.4 Sound absorption area

The equivalent sound absorption area A, of the empty room,

determined in one-third octave bands, shall not exceed the

values given in table 1.

Table 1 - Maximum equivalent sound absorption areas for room volume V = 200 m3

Equivalent sound absorption area, rri 6.5 6.5 6.5 7.0 9.5 13.0

I- Frequency. Hz i 125t_2j~ / 560 i 1000 i 2000 i 4ooO 1

4

IS:822591987 IS0 354-1985

If the volume V of the room differs from 200 m3, the values

given in table 1 shall be multiplied by the factor ( V/20012’3.

The graph of the equivalent sound absorption area of the

empty room versus frequency should be a smooth curve and

should have no dips or peaks differing by more than 15 % from

the mean of the values of both adjacent one-third octave

bands.

6.2 Test specimen

6.2.1 Plane absorbers

6.2.1.1 The test specimen shall have an area between 10 and

12 &. If the volume Vof the room is greater than 250 m3, the

normal test specimen area shall be increased by the factor

(V/2!5oP3.

NOTE - For the testing of materials with exceptionally small sound

absorption coefficients, it is recommended that test specimens with an

area larger than specified be used in order to obtain a significant

difference between the measured reverberation times r, and r,

(see 8.1.2).

6.2.1.2 The test specimen should be of rectangular shape

with a ratio of width to length between 0.7 and 1. It shall be

placed so that no part of it is closer than 1 m to any edge of the

boundary of the room. The edges of the test specimen should

preferably not be parallel to the nearest edge of the room.

6.2.1.3 The test specimen shall be mounted in accordance

with the relevant specifications provided by the producer or

with the application details provided by the user.

In the case of a test specimen directly mounted on a room sur-

face, the edges shall be totally and tightly enclosed by a frame

constructed from reflective material of rectangular cross-

section and, in general, of thickness not greater than 2 cm. The

frame shall not protrude above the surface of the test specimen. It shall be tightly sealed to the room surface on

which it is mounted.

In the case of a test specimen backed by an airgap, for instance

to simulate a suspended ceiling, sidewalls shall be constructed

perpendicular to the test surface. The sidewalls shall enclose

both the airgap and the test specimen edges, and shall be highly reflective.

NOTES

1 The measurement of the reverberation time of the empty room

should be made in the absence of the frame or the sidewalls of the test specimen.

2 As an alternative, in the case of test specimens backed by an

airgap, the test specimen can be mounted in a recess in one of the

boundaries of the reverberation room. It is, however, possible that this

method will not give the same results as the method specified.

6.2.2 Discrete sound absorbers

6.2.2.1 Discrete objects, for example chairs, persons, space

absorbers, shall be installed for test in the same manner as they

are typically installed in practice. For example, chairs or

freestanding screens shall rest on the floor, but they shall not

be closer than 1 m to any other boundary. Space absorbers

shall be mounted at least 1 m from any boundary or room dif-

fusers and at least 1 m from any microphone.

6.2.2.2 A test specimen should comprise a sufficient number

of individual objects (in general, at least three) to provide a

measurable change in the equivalent sound absorption area of

the room greater than 1 m*, but not more than 12 m*. If the

volume V of the room is greater than 250 m3, these values shall

be increased by the factor 12 ( V/25O)2’3.

Objects normally treated as individual objects should be ar-

ranged randomly, spaced at least 2 m apart. If the test

specimen comprises only one object, it should be tested in at

least three locations, at least 2 m apart, and the results aver-

aged.

6.2.2.3 If the test specimen comprises a given array of objects

(for example theatre chairs, noise absorber pads), they shall be

installed for test in this configuration. When testing groups of

seats with seated persons, the edges of the arrangement shall

be enclosed by reflecting material. This enclosure should have a

height of up to 1 m. In other cases, the height of the enclosure

should be adapted to the height of the test specimen.

6.2.3 Curtains

Curtains tested against walls can be treated as plane absorbers

(6.2.1) if closed, or as discrete absorbers (6.2.2) if open. In the

former case, the edges shall be enclosed. The requirements for

a minimum distance of 1 m from the walls or from the edges do

not apply in the case of curtains.

6.3 Temperature and relative humidity

The relative humidity in the room shall be greater than 40 %.

During a series of measurements of reverberation times T, and

r2 (see 8.12). the relative humidity and the temperature should

be as constant as possible and at least the conditions given in

table 2 should be satisfied.

Table 2 - Requirements for temperature and relative humidity during measurements

of T, and T,

Relative humidity Relative humidity Temperature during

range during all

( all measurements , Lower temperature

measurements within within I limit / L 1

40 up to 60

% 3 % 3 “C 10 ‘>C

> 60 % 5% 5 “C 10 oc I

5

IS : 8225 - 1987 IS0 354 - 1985

The test specimen should be allowed to reach equilibrium with

respect to temperature and relative humidity in the room before

tests are carried out.

NOTE - Additional correction of the results for the equivalent absorp-

tion area A in accordance with 8.12, allowing for the energy attenu-

ation in the air, may be applied, but the correction shall not exceed

0,5 n? of the equivalent sound absorption area. The method of correc-

tion and the origin of the correction data should be given in the test

report.

7 Test procedure

7.1 Generation of sound field

The sound in the reverberation room shall be generated by one

or more loudspeakers the radiation pattern of which is as non-

directional as possible. For frequencies below 300 Hz,

measurements should be made with a sound source in at least

two successive positions (at least 3 m apart) or with an

equivalent multiple source arrangement, the sources not

sounding simultaneously unless driven by separate (incoherent-

ly related) noise sources.

The test signals shall consist of band-limited noise having a

continuous frequency spectrum with a bandwidth of at least

one-third octave.

The level of the steady exciting signal before decay shall be suf-

ficiently above the level of the background noise to permit

evaluation of the decay curves as specified in 7.2.2.

The exciting signal before being switched off should be suf-

ficiently long to produce a time-constant sound pressure level

in-the room.

NOTES

1 !f a signal with a bandwidth greater than one-third octave is used,

!ong reverberation times in adjacent frequency bands can influence the

lower part of the decay curve. If the reverberation times in adjacent

bands differ by more than a factor of 1.5, the reverberation times for

those bands with the shortest reverberation times should be measured

individually using one-third octave filtering of the sound source.

2 Use of wide-band noise and a computer-controlled real-time

analyser to make simultaneous measurements for all frequency bands

IS acceptable, subject to the factors mentioned in note 1. For these

measurements with wide-band noise, the average sound spectrum in

the room should approximate pink or white noise with differences in

sound pressure level less than 6 dB between adjacent one-third

octaves.

7.2 Measurement of rever~beration time

7.2.1 Receiving equipment

The receiving equipment shall consist of one or more micro-

phones which are as non-directional as possible, the necessary

amplifiers, filters and a measuring system for reverberation

time.

The recordings shall be made with at least three microphone

positions at least i./2, apart, where E. is the wavelength of

sound for the centre frequency of the frequency band of

interest.

Only one microphone shall be used at a time. The microphoneS

shall be at least 1 m from the test specimen, 1 m from room

surfaces or diffusers and 2 m from the sound source(s).

The recording system shall be a level recorder or any other ade-

quate equipment for determining the average slope of the

decay curve of the corresponding reverberation time.

.The apparatus for recording (and displaying and/or evaluating)

the decay in sound pressure level may use

a) exponential averaging, with a continuous curve as out-

put; or

b) exponential averaging, with successive discrete sample

points from the continuous average as output; or

cl linear averaging, with successive discrete linear

averages as output, in some cases with pauses of con-

siderable duration between determinations of averages.

The averaging time of an exponential averaging device (or ap-

proximate equivalent; see note 2) shall be less than, but as

close as possible to, T/20.

The averaging time of a linear averaging device shall be less

than T/7.

For apparatus in which the decay record is formed as a succes-

sion of discrete points, the time interval between points on the

record shall be less than 1,5 times the averaging time of the

device.

In all cases where the decay record is to be evaluated visually,

the time scale of the display should be adjusted so that the

slope of the record is as close to 45’ as possible.

NOTES

1 The averaging time of an exponential averaging device is equal to

8.69 divided by the decay rate, in decibels per second, of the device.

2 Commercial level recorders in which the sound pressure level is

recorded graphically as a function of time are approximately equivalent

to exponential averaging devices.

3 When an exponential averaging device is used, there is little advan-

tage in setting the averaging time to very much less than T/20. When a

linear averaging device is used, there is no advantage in setting the in-

terval between points to very much less than T/7. In some sequential

measurement procedures, it is feasible to reset the averaging time ap-

propriately for each frequency band. In other procedures, this is not

feasible, and an averaging time or interval chosen as above with

reference to the smallest reverberation time in any band should be

used for measurements in all bands.

One-third octave filters shall be included in the receiving equip-

ment. The discrimination characteristics of the filters shall be in

accordance with IEC Publication 225.

7.2.2 Evaluation of decay curves

The reverberation time shall be evaluated from the averaged

slope of the decay curve over a convenient range, beginning

6

IS : 8225 - 1987 IS0 354 - 1985

about 0.1 s after the sound source has been switched off, or

from a sound pressure level a few decibels lower than that at

the beginning of decay. The range used shall neither be less

than 20 dE3 nor shall it be so large that the observed decay can-

not be approximated by a straight line. The bottom of this

range shall be at least 15 dB above the combined background

noise level of the reverberation room and the recording equip-

ment for each one-third octave band.

A decay may be described as approximately straight if

measurements of the slope of two subsections of the curve

(each covering a range of at least 10 dB, with one extending to

a sound pressure level at least 10 dB lower than the other) do

not differ by more than 10 %.

8.1.2 Calculation of A,, A, and A

8.1.2.1 The equivalent sound absorption area A,, in square

metres, of the empty reverberation room, shall be calculated

using the formula

55.3 v

A1 =cT 1

where

V is the volume, in cubic metres, of the empty reverber~

ation room;

For each combination of microphone and loudspeaker position,

and for each one-third octave band, an ensemble averaging

procedure, involving the superposition of several repeated ex-

citations of the room, may also be used to obtain a single decav

curve from which the reverberation time can be evaluated.

c is the velocity of sound in air, in metres per second;

T, is the reverberation time, in seconds, of the empty

reverberation room.

NOTE - For temperatures in the ranqe 15 to 30 OC, the velocltv of

sound in air, c, in metres per second, can be calculated from the for

mula

7.3 Frequency ranges for measurements

The measurements shall be carried out at the following centre

frequencies, in hertz, from the one-third octave band series:

100 125 160 200 250 .315 400 500 630 800 1000 1 250

1600 2000 2 560 3150 4000 5000

7.4 Number of measurements

(‘ = 331 + 0.6 I

where / is the air temperature, in degrees Celsius.

8.1.2.2 The equivalent sound absorption area A,, in square

metres, of the reverberation room containing a test specimen,

shall be calculated using the formula

55,3 v A, = __

CT2

where

The minimum number of -measurements required for each fre-

quency band is:

a) twelve decays from 100 to 250 Hz (for example, two

each of six sound source/microphone combinations);

c and V have the same meanings as in 8.1.2.1;

T2 is the reverberation time, in seconds, of the reverber-

ation room after the test specimen has been introduced.

b) nine decays from 315 to 800 Hz (for example, three

each of three sound source/microphone combinations);

8.1.2.3 The equivalent sound absorption area A, in square

metres, of the test specimen, shall be calculated using the for-

mula

cl six decays from 1 000 to 5 000 Hz (for example, two

each of three sound source/microphone combinations).

8 Expression of results c, V and Tl have the same meanings as in 8.1.2.1;

8.1 Method of calculation T, has the same meaning as in 8.1.2.2.

NOTE - The area of room surface covered by the test specimen is not

8.1.1 Calculation of reverberation times Tl and T2 taken Into account by thts formula (see annex 6).

The reverberation time of the room in each frequency band is 8.1.3 Calculation of aS (see also annex BI

expressed by the arithmetic mean of the total number of

reverberation time measurements made in that frequency band. The sound absorption coefficient aa of a plane absorber shall

be calculated using the formula

The mean reverberation times T, and T2 in each frequency

band shall be calculated and expressed to at least two decimal

places.

A

us = ~- s

7

IS : 8225 - 1987 IS0 354 - 1985

where

A is the equivalent sound absorption area, in square

metres, calculated in accordance with 8.1.2.3;

S is the area, in square metres, of the test specimen.

8.1.4 Calculation of equivalent sound absorption area of discrete absorbers

For discrete absorbers, the result should generally be expressed

as equivalent sound absorption area per object, which is deter-

p mined by dividing A by the number of objects tested.

For a specified array of objects, the result should be given as

equivalent sound absorption area of the whole configuration.

The equivalent sound absorption area of a test specimen

should be rounded to 0.1 m* and the sound absorption coef-

ficient to 0,Ol.

NOTE - This way of rounding results leads to the presentation of smooth curves in the graphs. It should, however, be borne in mind that the precision of the results may be less than the above decimal round- ing limits might imply.

In the graphical presentation, the points of measurement

should be connected by straight lines, the abscissa giving the

frequency on a logarithmic scale and the ordinate showing the

equivalent sound absorption area or sound absorption coeffi-

cient on a linear scale. The ratio of the ordinate distance from

A = 0 to A = 10 m2, or from as = 0 to es = 1, to the

abscissa distance corresponding to 5 octaves, should be 2 : 3.

8.2 Precision

The precision of the test procedure can be defined by its

repeatability (see 3.5) and reproducibility (see 3.61, as described

in IS0 5725.

Results that display extreme dips or peaks that cannot be ex-

plained by physical characteristics of the material under test or

its mounting should be indicated as doubtful.

9 Test report Comparison tests involving a number of reverberation rooms

have given a rough assessment of reproducibility of sound ab-

sorption coefficient measurements as shown in the figure. The test report shall make reference to this International Stan-

dard and shall include the following information :

for an inter-laboratory test, as specified in IS0 5725.

NOTE - If the sound absorption coefficient shows steep variations as a function of frequency, the reproducibility may significantly exceed the values shown in the figure.

For the time being, insufficient information on repeatability is

available to give an assessment of it in this International Stan-

dard. For the purpose of checking the repeatability within a

single laboratory, an estimation may be made using the method

described in annex C. Reliable figures on repeatability and

reproducibility can be found only by following the procedure

a)

microphone- and sound source positions;

the name of the organization that performed the test;

b) the date of test;

c) the description of the test specimen, its surface area S,

mounting and position in the reverberation room, preferably

by means of drawings:

d) the shape of the reverberation room, its diffusion treat-

ment (the number and size of diffusers) and the number of

8.3 Presentation of results

For all frequencies of measurement, the following results shall

be reported, presented in the form of a table and as a graph:

a) for plane absorbers, the sound absorption coefficient

as;

b) for single objects, the equivalent sound absorption area

per object;

c) for a specified array of objects, the equivalent sound

absorption area of the whole configuration.

e) the dimensions of the reverberation room, its volume I/

and its total surface area (walls, floor and ceiling), S,;

f) the type of noise used;

g) the temperature and relative humidity;

h) the mean reverberation times 7-, and T2 at each fre-

quency;

j) the results, reported in accordance with 8.3;

k) the repeatability, if calculated (see annex Cl.

IS : 8225 - 1987 IS0 354 - 1985

Ic

Frequency, Hz

Figure - Assessment of reproducibility R of as for a) sample 1 having a high absorption coefficient Ias = 1.00). and

b) sample 2 having a low absorption coefficient (as = 0.05) in all one-third octave bands

9

IS : 8225 - 1987 IS0 354 - 1985

Annex A

Diffusivity of the sound field in the reverberation room

(This annex forms an integral part of the standard.)

A.1 Diffusers

An acceptable diffusivity can be achieved by using fixed dif-

fusers and/or rotating vanes. Ideally, these diffusing elements

should be damped sheets with low sound absorption and with

a mass per unit area of at least 5 kg/m*. Diffusers of different

sizes, ranging from approximately~0.8 to 3 m* in area (for one

side) are recommended. The sheets may be slightly curved and

should be oriented at random and positioned throughout the

room.

If rotating vanes are used, the decay repetition frequency and

the frequency of rotation of the vane should not be in the ratio

of small whole numbers.

A.2 Check of diffusivity

Select a suitable test specimen, i.e. a sample, 5 to 10 cm thick,

of a homogeneous, porous absorbing material which, under

optimum conditions, has a sound absorption coefficient greater

than 0,9 over the frequency range from 500 to 4~000 Hz. (Cer-

tain glass-wools, rockwools or polyurethane foams meet this

criterion.)

Mount a test specimen in accordance with 6.2.

Perform sound absorption measurements on the test specimen

as follows :

a) with no diffusers;

b) with a small number of stationary diffusers fapproxi-

mately 5 m* in area); and

c) with increasing quantities of stationary diffusers, in

steps of approximately 5 m* in area.

For each set of measurements, calculate the mean value of the

sound absorption coefficients, in the range from 500 to

4 000 Hz, and plot these values against the number of diffusers

used in each case.

It will be seen that the mean sound absorption coefficient ap-

proaches a maximum and thereafter remains constant with in-

creasing numbers of diffusers. The optimum number of

stationary diffusers is that at which this constant value is first

attained.

NOTES

1 From experience, it has been found that, in rectangular rooms, the

area (both sides) of diffusers required to achieve satisfactory diffusion

is approximately 15 to 25 % of the total surface area of the room.

2 If rotating vanes are used, the resulting diffusion should be proved

to be equivalent to that achieved by the procedure described above.

Annex B

Explanatory remarks on the formulae in 8.1.2.3 and 8.1.3

(This annex does not form an integral part of the standard.)

For normal absorbing materials, there is a small error in the calculated value due to neglecting the absorption of the area covered by

the test material, the calculated value being slightly too low.

A greater error would, however, certainly result if the sound absorption coefficient of the covered area was calculated from the

reverberation time of the empty room, because this time depends not only on the absorption of the walls, but probably more on that

of the other objects (such as doors, loudspeakers, light fittings), by dissipation of sound energy in the air and by vibrations of the walls

and ceiling which are not hindered if they are covered with absorbing material.

70

IS : 8225 - 1987 I-SO 354 - 1985

Annex C

Determination of repeatability

(This annex does not form an integral part of the standard.)

Repeatability is determined by repeated tests made within a short interval of time on the same test specimen following the procedure

specified in this International Standard, as used within the laboratory (using the same number of microphone positions, excitations of

the room, recordings of decay curves and the same evaluation of the reverberation times for each test).

At least five tests should be made under conditions which are as stable as oossible.

Special care should be taken to ensure that the test specimen does not change due to the repeated operations of mounting and dis-

mounting between tests.

The repeatability r within the laboratory can be estimated from the formula

where

0, is the result of measurement i;

G is the arithmetic mean of the set of n measurements: crl.. rr,... CY,,;

I is the factor derived from Student’s distribution for a probability level of 95 % and the appropriate number of degrees of

freedom (see table 3).

Table 3 - Factor “I”

1’ = ,I - 1 4 5 1 6 7 8 9 10 20 00

t 2.78 2.57 2,45 2.37 2.31 2,26 2,23 2.09 1,96

NOTE - Determmations of repeatability should preferably be carried out on materials with sound-absorbing coefficients of different magnitude. As a minimum, two repeatability tests should be carried out, one of them using a highly absorbing material.

11 Reprography Unit, BIS; New Delhi, India